1. Field of the Invention
The present invention relates to a variable speed geared motor constructed by combining an inverter motor controllable in rotational speed by inverter drive with a gear reducer.
2. Description of the Prior Art
A geared motor constructed by integrating a gear reducer with a motor is widely utilized as a power transmitting basic unit for driving every industrial and general machinery at an optimum rotational speed and torque.
FIG. 4 is a developed sectional view illustrating a structure of a geared motor 10 in the prior art. In FIG. 4, reference numeral 20 denotes a motor and reference numeral 30 denotes a gear reducer. An increase in output torque of the geared motor 10 can be attained by enlarging a reduction ratio i of the gear reducer 30 for the same motor 20.
Accordingly, as shown in FIG. 4, a general geared motor is adapted to accommodate a two-stage or three-stage gear train.
Further, in order to compactly accommodate the two-stage or three-stage gear train, a first-stage pinion 21 is formed by directly cutting a motor shaft 11 or press-fitted with the motor shaft 11.
Further, an attempt has been made to more compactly construct the geared motor by improving a bearing metal of an output shaft.
As shown in FIG. 6 according to the standard of JIGMA 212-01 by Japan Industrial Gear Manufactures Association, for example, it is known that reduction ratios of 6.3-50 are obtained in case of a two-stage gear train and reduction ratios of 31.5-315 are obtained in case of a three-stage gear train.
On the other hand, a gear train providing reduction ratios of 8 and less has rarely been manufactured as a one-stage gear train in accordance with a series of reduction ratios by the standard of JIGMA 211-01 as shown in FIG. 5.
Meanwhile, a variable speed drive by a three-phase AC squirrel-cage motor of variable frequency inverter drive has been put into practical application through a development of electronic technology since the years of 1980. Further, a variable speed geared motor has also been put into practical application by combining the above motor with a gear reducer to obtain a large output torque at low speeds and a variable rotational speed.
In the inverter drive, the motor is driven at the frequencies 1/N-1/1 times a commercial frequency (50 Hz or 60 Hz) (e.g., at the frequencies 5 Hz-50 Hz in case of N=10 in an area where the commercial frequency is 50 Hz), thereby ensuring a speed change region of 1:N.
In such circumstances, it is general that the motor of inverter drive is not combined with a gear reducer having the variable reduction ratio of N or less.
Actually, there exceptionally exists a marketed article having a reduction ratio of 5 lower than the reduction ratio of 10.
There is considered to be primarily due to the following reasons.
(1) In an inverter motor, any arbitrary one of the reduction ratios of about 1-10 can be attained by the inverter motor itself. Accordingly, in the case of requiring the reduction ratios over 10, the combination of the inverter motor with a gear reducer becomes really significant.
(2) If a gear reducer having a reduction ratio of 8 or less (particularly less than 6.3) is conventionally manufactured, it will become a one-stage gear reducer.
As a result, a distance between a motor shaft and an output shaft becomes equal to a center distance of gears, so that the center of a variable speed geared motor becomes too high (or too low) and a general construction thereof cannot be made compact.
However, the variable speed geared motor having the reduction ratios set in accordance with the above conception does not always sufficiently meet a certain need at present.
For example, in the application to driving of a pump or a high-speed conveyor, a relatively high rotational speed region of 200-900 rpm is used as an ordinary controlled rotational region. However, in this rotational region, the variable speed geared motor mentioned above in the prior art is not economical enough to efficiently utilize an output of the inverter motor.
This is considered to be due to the following first reason. That is, since the output shaft (coupling) of the variable speed geared motor is designed on the basis of an output torque and a radial load, it is required to be adapted for "high reduction ratio" with a large output torque. As a result, a diameter of the output shaft becomes large, and it is therefore necessary to adopt a large coupling or the like. Accordingly, the variable speed geared motor causes "poor economy".
A second reason for the "poor economy" is considered to be as follows:
FIG. 7 is a graph showing the relation between a rotational speed of an output shaft of a conventional variable speed geared motor and an output torque thereof.
The output torque is shown with a rated torque of an inverter motor assumed to be 100%. In FIG. 7, reference character A denotes an output torque range of the inverter motor itself; B denotes an output torque range of the geared motor combined with a reduction ratio i=5; and C denotes an output torque range of the geared motor combined with a reduction ratio i=10. These output torque ranges are shown with an efficiency of a gear reducer assumed to be 90%.
FIG. 8 is a graph showing the relation between a rotational speed of an output shaft of a conventional variable speed geared motor and an allowable input capacity thereof. The allowable input capacity is shown with a rated capacity (kW) of an inverter motor assumed to be 100%. In FIG. 8, reference character E denotes an allowable input capacity range of the inverter motor itself, G denotes an allowable input range of the geared motor combined with a reduction ratio i=5; and F denotes an allowable input capacity range of the geared motor combined with a reduction ratio i=10.
As apparent from FIG. 8, in a relatively high rotational speed region (e.g., rotational speeds higher than 360 rpm), the allowable input capacity of the conventional variable speed geared motor is very low with respect to the rated capacity of the inverter motor, and a capacity of the inverter motor itself cannot be sufficiently exhibited. Accordingly, a larger inverter motor (a single member) and an inverter are required in some application, thus causing the "poor economy".
In view of this circumstance, another driving method is disclosed in Japanese Patent Laid-open Publication No. 60-26849 such that a so-called mechanical transmission such as a frictional continuous transmission constructed by combining a lubricating oil traction with a differential planetary mechanism is used in a relatively high rotational speed region.
An output torque characteristic and an allowable input capacity characteristic of the mechanical transmission are shown by reference character D in FIG. 7 and reference character H in FIG. 8, respectively.
The combination of this mechanical transmission with a general-purpose squirrel-cage motor can provide an intermediate characteristic between a constant horsepower characteristic and a constant torque characteristic.
Accordingly, a large torque can be generated in a low rotational speed region. Thus, the mechanical transmission has a characteristic more advantageous than the variable speed geared motor in one aspect. However, since a speed changing operation itself of the transmission must be mechanically performed, a control device for remotely performing this operation of the transmission is additionally required. Accordingly, this driving method is not so suitable for a system using a pump or a high-speed conveyor which is required to collectively control many transmissions in one position.